Linux时间操作(time、gettimeofday)

自：http://blog.chinaunix.net/space.php?uid=24148050&do=blog&id=320294

一、time函数

#include <time.h>

time_t time(time_t *calptr);

返回距计算机元年的秒数
一旦取得这种以秒计的很大的时间值后，通常要调用另一个时间函数将其变换为人们可读的时间和日期

#include <time.h>
//calendar time into a broken-down time expressed as UTC
struct tm *gmtime(const time_t *calptr);

//converts the calendar time to the local time, taking into account the local time zone and
//daylight saving time flag
struct tm *localtime(const time_t *calptr);

//converts it into a time_t value
time_t mktime(struct tm *tmptr);

struct tm { /* a broken-down time */
int tm_sec; /* seconds after the minute: [0 - 60] */
int tm_min; /* minutes after the hour: [0 - 59] */
int tm_hour; /* hours after midnight: [0 - 23] */
int tm_mday; /* day of the month: [1 - 31] */
int tm_mon; /* months since January: [0 - 11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday: [0 - 6] */
int tm_yday; /* days since January 1: [0 - 365] */
int tm_isdst; /* daylight saving time flag: <0, 0, >0 */
};

char *asctime(const struct tm *tmptr);

char *ctime(const time_t *calptr);

asctime()和ctime()函数产生形式的26字节字符串，这与date命令的系统默认输出形式类似：
Tue Feb 10 18:27:38 2004/n/0




二、gettimeofday函数得到更精确的时间

#include <sys/time.h>

int gettimeofday(struct timeval *restrict tp, void *restrict tzp);

第二个形参是基于平台实现的，使用的时候最好用NULL

struct timeval{
time_t tv_sec; /*** second ***/
susecond_t tv_usec; /*** microsecond 微妙***/
}

1秒＝1000毫秒，
1毫秒＝1000微秒，
1微妙＝1000纳秒，
1纳秒＝1000皮秒。
秒用s表现,毫秒用ms,微秒用μs表示，纳秒用ns表示，皮秒用ps表示。

三、内核时间

内核有两个重要的全局变量:
unsigned long jiffies;
timeval xtime ;

jiffies 是记录着从电脑开机到现在总共的"时钟中断"的次数。
文件linux-2.6.24/kernel/timer.c
void do_timer(unsigned long ticks)
{
jiffies_64 += ticks;
update_times(ticks);
}

xtime 是从cmos电路或rtc芯片中取得的时间，一般是从某一历史时刻开始到现在的时间。
这个就是所谓的"墙上时钟walltimer"，通过它可计算得出操作系统需要的日期时间，它的精确度是微秒。

xtime第一次赋值是在系统启动时调用timekeeping_init或time_init进行的
再调用read_persistent_clock进一步调用get_rtc_time得到的

PS：在/proc/uptime里面的两个数字分别表示:
the uptime of the system(seconds),
and the amount of time spent in idle process(seconds).

四、代码示例

“UTC时间字符串”与 “time函数返回值”互换

int64_t TimeToUTC(char *time)

{

struct tm temp1;

int rc;

int year;

int mon;

int day;

int hour;

int min;

int sec;

rc = sscanf(time, "%4d-%2d-%2d %2d:%2d:%2d",

&(temp1.tm_year),

&(temp1.tm_mon),

&(temp1.tm_mday),

&(temp1.tm_hour),

&(temp1.tm_min),

&(temp1.tm_sec));

sscanf(time, "%4d-%2d-%2d %2d:%2d:%2d",

&year,

&mon,

&day,

&hour,

&min,

&sec);

if((rc<6)

|| (temp1.tm_year<1900) || (temp1.tm_year>2100)

|| (temp1.tm_mon<1) || (temp1.tm_mon>12)

|| (temp1.tm_mday<1) || (temp1.tm_mday>31)

|| (temp1.tm_hour<0) || (temp1.tm_hour>23)

|| (temp1.tm_min<0) || (temp1.tm_min>59)

|| (temp1.tm_sec<0) || (temp1.tm_sec>59))

{

return -1;

}

temp1.tm_mon -= 1;

temp1.tm_year -= 1900;

struct tm temp3;

time_t temp2 = mktime(&temp1);

if (temp2 == -1){

return -1;

}

else{

localtime_r(&temp2, &temp3);

if (!((mon == (temp3.tm_mon+1))

&& (day == temp3.tm_mday)

&&(year == (temp3.tm_year+1900)))){

return -1;

}

else{

return temp2*1000000LL;

}

}

}

void UTCToTime(int64_t utc, char * clock)

{

struct tm temp1;

time_t sec;

float msec;

int rc;

sec = utc/1000000;

msec = utc/1000000.0-sec;

localtime_r(&sec, &temp1);

temp1.tm_year += 1900;

temp1.tm_mon += 1;

rc = sprintf(clock, "%04d-%02d-%02d %02d:%02d:%02d",

temp1.tm_year,

temp1.tm_mon,

temp1.tm_mday,

temp1.tm_hour,

temp1.tm_min,

temp1.tm_sec);

clock[rc] = '/0';

}

生成Date

static char * g_weekstr[7] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

static char * g_monthstr[12] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun",

"Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };

/*generate the current date string*/

time_t now = time(NULL);

struct tm tt;

gmtime_r( &now, &tt );

char timebuf[64];

snprintf( timebuf, sizeof(timebuf),

"%s, %02d %s %d %02d:%02d:%02d GMT",

g_weekstr[tt.tm_wday], tt.tm_mday,

g_monthstr[tt.tm_mon], tt.tm_year + 1900,

tt.tm_hour, tt.tm_min, tt.tm_sec );

------------------------------ 华丽的分割线 ------------------------------------

关于scanf的返回值
Both scanf and wscanf return the number of fields successfully converted
and assigned; the return value does not include fields that were read but
not assigned. A return value of 0 indicates that no fields were assigned.
The return value is EOF for an error or if the end-of-file character or the
end-of-string character is nocountered in the first attempt to read a character.

如：scanf("%d%d", &a, &b);
如果a和b都被成功读入，那么scanf的返回值就是2
如果只有a被成功读入，返回值为1
如果a和b都未被成功读入，返回值为0
如果遇到错误或遇到end of file，返回值为EOF。

void main()

{

int a;

int b;

int c;

int x;

printf("请输入三个整数：");

x=scanf("%d%d%d",&a,&b,&c);

printf("%d/n%d/n",a,x);

}

输入三个整数：5 6 7则x的值为3；

输入5 6 d（即给c 赋值不正确）则x的值为2；

输入5 t d（即给b和c 赋值不正确）则x的值为1；

scanf()的返回值对我们来说也很有用的，例如可使用if(scanf("%d,%d",&a,&b)＝＝2)这样语句来判断是否正确的给所有的变量赋值了，正确的话才能使用这个变量与运算，这样才能提高我们代码的安全性。